Adeno-associated virus (AAV) is a 4.7 kb, single stranded DNA virus that contains two open reading frames, rep and cap. The first gene encodes four proteins necessary for genome replication (Rep78, Rep68, Rep52, and Rep40), and the second expresses three structural proteins (VP1-3) that assemble to form the viral capsid. As its name implies, AAV is dependent upon the presence of a helper virus, such as an adenovirus or herpesvirus, for active replication. In the absence of a helper it establishes a latent state in which its genome is maintained episomally or integrated into the host chromosome. To date, numerous AAV serotypes in humans have been identified.
In 1989 a recombinant AAV2 (rAAV) gene delivery vector system was first generated, and vectors based on AAV have subsequently been shown to offer numerous major advantages. First, vectors based on AAV are extremely safe, since wild-type AAV is nonpathogenic and has no etiologic association with any known diseases. In addition, AAV offers the capability for highly efficient gene delivery and sustained transgene expression in numerous tissues, including muscle, lung, and brain. Furthermore, AAV has enjoyed success in human clinical trials.
Despite this success, vector design problems remain. One major concern is the fact that much of the human population has already been exposed to various AAV serotypes, and as a result a significant fraction of any future patient population harbors neutralizing antibodies (NABs) that block gene delivery. Additional problems with rAAV vectors include limited tissue dispersion for serotypes that employ heparan sulfate as a receptor (AAV2 and 3), poor infection of non-permissive cell types such as stem cells, challenges with high efficiency targeting of gene delivery to selected cell populations, and a finite transgene carrying capacity.
There is a need in the art for improved AAV vectors that can infect cells that are non-permissive for AAV.